In cellular mobile systems like GSM (Global System for Mobile communication) (but also in cellular public land mobile networks according to its successors like UMTS (Universal Mobile Telecommunications System) or E-UTRAN (Evolved UMTS Terrestrial Radio Access Network)/LTE (Long Term Evolution)), the so-called “PLMN Selection” (PLMN=Public Land Mobile Network) is performed by the user equipment (UE) as defined in [3GPP TS 22.011]. Subsequently this selected PLMN becomes the so-called “registered public land mobile network” or “registered PLMN” (RPLMN) of the terminal (user equipment). Details can be found in [3GPP TS 23.122] for cellular mobile systems according to the 3GPP standard.
While a user equipment is kept registered on the registered public land mobile network, it can originate mobile originated calls (MOC) and receive mobile terminated calls (MTC). This is the default state a normal UE is in, in order to provide service to the customer.
Typically in cellular mobile systems each user equipment has only a single public land mobile network to which is has been registered (single registered public land mobile network) and to and from which services like MOC and MTC are provided.
As a single public land mobile network can provide typically all service other PLMNs can also offer. The main reason to select a certain public land mobile network or operator for a contract is—besides quality of the network—the price of the service being charged. As there is a desire of customers to optimise payments in the recent past so-called “Dual-SIM/Dual-Standby” (DSDS) devices came to the markets. Such user equipments are, e.g., known from the publication US 2011/0217969 A1.
These DSDS terminals or user equipments consist basically of two terminals in a single terminal housing. For price reasons, these terminals do not consist of two fully independent terminals in a single housing, but manufactures of such devices aim to reuse as much as possible of the device to allow independent operation.
For the end customers such devices allow the operation of a single device in two independent cellular public land mobile networks. Therefore, it is possible to use two independent subscriptions also with two different operators. This provides multiple possibilities to the customer, like:                Being reachable with two independent telephone numbers (e.g. a private and a business one);        Allow two different contracts (e.g. a private one for private calls and a business one)        In case of abroad roaming, a local subscription to receive and place cheap local (national) calls and the normal international subscription to be reachable via the normal number;        Different types of contracts with the different public land mobile networks, such as a voice call contract with the first public land mobile network (PLMN 1) and a data contract with the second public land mobile network (PLMN 2);        A single terminal with two subscriptions in different regions (no need to swap SIMs) when no roaming between the operators is in place.        It is possible with a single DSDS device or DSDS user equipment to use more than a single radio access technology concurrently on the same or different PLMNs.        
Technically, such DSDS user equipments or DSDS mobile devices consist of two subscriber identity modules (SIM) inserted in two terminal devices, while both of the baseband devices are sharing a single radio frequency front-end for cost reasons.
In order to save power consumption of mobile devices (user equipments), cellular systems (i.e. cellular public land mobile networks)—like any of the 3GPP systems—implement the concept of discontinuous reception (DRX). This concept allows battery powered devices like mobile phones as user equipments to optimise the power consumption by falling into a so-called “sleep mode” most of the time while no communication is ongoing. The systems typically have defined “wakeup-times” during which the terminal listens to the Paging Channel (PCH) which notifies terminals about MTC (Mobile Terminated Calls). These wake-up times are defined for example for UMTS in [3GPP TS 25.304] and E-UTRAN/LTE in [3GPP TS 36.304].
Terminals (user equipments) are configured to follow a predefined interval, called DRX cycle, i.e. a time interval, which defines times during which the terminal is required to be pageable via the PCH. Typical time intervals or cycles are ranging between 0.320 s and 2.56 s. E.g. by defining a DRX cycle of 1.28 s, the terminal needs to wake up every 1.28 s to receive a potentially mobile terminating call which is indicated via the paging channel PCH. If the terminal in a given DRX occasion is not addressed on the PCH, it will fall back to sleep and wake up after another period of the time interval, i.e.—in this example—after another 1.28 s. The given reference specifications also define a formula how a UE has to calculate it's assigned DRX cycle using parameters from the broadcast channels of the given cellular public land mobile network and other input parameters, such as the SIM International Mobile Subscriber Identity (IMSI) of the user equipment.
The network is aware of the timing of the user equipment DRX cycle and thus will page the terminal on the PCH only at those times when the terminal is woken up and listens to the PCH.
This DRX concept has also the benefit that the paging rate for terminals can be spread in time as not all terminals follow the same DRX cycle and thus will not be paged at the same point in time. This concept allows efficient utilisation of the PCH channel resources.
DSDS terminals in contrast to a normal single SIM UE have two independent SIM cards (also with individual IMSIs). Hence in order to be pageable they also need to follow two independent DRX cycles (typically calculated based on the individual IMSIs).
The subscriptions of the two SIM cards can either relate to the same public land mobile network or to different public land mobile networks.
As especially for the case of independent public land mobile networks (i.e. the first public land mobile network is different to the second public land mobile network), no coordination of the individual DRX cycles is possible due to the fact that the public land mobile networks are not coordinated (i.e. there is an absolute time offset between the beginning of a radio frame (or other timing structure) in the first public land mobile network and the beginning of a radio frame (or other timing structure) in the second public land mobile network), the user equipment tracks both of the DRX cycles on its own. This becomes especially problematic while a DSDS user equipment is in data communication with the second public land mobile network while it still needs to ensure to follow the DRX cycle for reading the paging channel of the first public land mobile network.
With conventional DSDS terminals or user equipments, typically the user equipment autonomously tunes its radio front-end at the time intervals implied for the individual DRX cycles from the second public land mobile network to the first public land mobile network in order to ensure that MTCs from the first PLMN are not missed. In effect this means that for a short time, the user equipment or terminal cannot receive the data transmission from the second public land mobile network while reading the paging channel of the first public land mobile network.
As modern cellular data systems like Highspeed Data Packet Access (HSPA) or Long Term Evolution (LTE) are so-called “fully scheduled systems using a shared channel for a range of active terminals” this also becomes problematic in terms of system efficiency.
While the second public land mobile network schedules the terminal according to the rules defined by the second public land mobile network, the user equipment autonomously tunes away from the data reception from the second public land mobile network to receive the paging channel of the first public land mobile network. By doing so, data being sent to the user equipment by the second public land mobile network is lost and needs to be retransmitted during the next time the user equipment again listens to the second public land mobile network.
Besides loss of radio system efficiency this behaviour might also have a quite negative impact on the user experience. Especially if the data loss is such high that the TCP/IP retransmission behaviour is triggered resulting in a slow start of the TCP/IP connection.